1. Field of the Invention
The invention relates to the stabilization of the optical output characteristics of a semiconductor laser and, in particular, to a device for driving a semiconductor laser based on a feed-back controlling technique so as to eliminate the flow of over-current in the laser.
2. Description of the Related Art
Recently, semiconductor laser diodes have been proposed as an electro-optical converter for use in optical communication systems to convert digital or analog electrical signals into optical signals. A problem in such application is that the optical output characteristics of a semiconductor laser change with temperature.
To compensate for temperature changes in the laser output, it is required that a drive signal should be effectively controlled with respect to the amplitude of a bias current applied to the laser. In the conventional systems, however, when the laser diode is switched between a waiting mode and a normal oscillation mode, the above technical requirements cannot be satisfied sufficiently. This results in that rush current flow undesirably in the laser, when it is set in the waiting mode or when drive power is turned on to render the diode operative. In such a case, abnormal increase in the optical output level of the laser cannot be compensated effectively.
In Japanese Patent Disclosure (KOKAI) No. 59-145587, there is proposed to use a zener diode in a laser drive circuit, to thereby restrict the maximum level of the drive current to be supplied to the laser diode. With such an arrangement, however, since the maximum level of the optical laser output is simply limited to the fixed value, if such a value is too low, the operation range of the feed-back control loop for the laser output stabilization is narrowed, which leads to the degradation of the laser stabilizing performance.